1. Field of the Invention
The present invention relates to a solar cell module, and particularly to a solar cell module in which the light-receiving surface and/or non-light-receiving surface of a photovoltaic element are/is sealed by an organic polymeric resin.
2. Related Background Art
In recent years, there has been growing awareness of ecological problems all over the world. In particular, a severe concern has been rising over the earth-warming phenomenon caused by the emission of CO2, and the demand for clean energy has been further increasing. It can be said at present that a solar cell is a promising clean energy source because it is safe and easy to handle.
FIG. 5 is a schematic cross-sectional view showing the basic structure of a solar cell module. In FIG. 5, numeral 501 denotes a photovoltaic element, 502 a sealer resin, 503 a front surface member, and 504 a back surface member. Sunlight is incident upon the light-receiving surface of the photovoltaic element 501 by penetrating the front surface member 503 and the sealer resin 502 and converted into electric energy. Electricity generated is outputted from output terminals (not shown).
The photovoltaic element cannot withstand being directly used under a severe outdoor condition, because the photovoltaic element itself is susceptible to corrosion and easily broken by a shock from an external source. Therefore, the photovoltaic element must be covered and protected by a covering material. The most generally, this is done by a method in which the photovoltaic element is sandwiched and laminated between a transparent front surface member having weather resistance, such as glass and a fluorocarbon resin film, and a back surface member having excellent moisture resistance and electrical resistance properties, such as an aluminum-laminated teddler film and a polyester film.
As a conventional sealer resin for the solar cell, polyvinyl butyral and an ethylene-vinyl acetate copolymer (EVA) have been mainly used. Of these, the crosslinkable composition of an EVA has excellent properties such as heat resistance, weather resistance, transparency and cost efficiency and is the most popular sealer resin for the solar cell at present.
The solar cell module is required to have high degree of durability because It is used outside over a long period of time. To say nothing of the durability of the photovoltaic element itself, the covering material is also required to have excellent weather resistance and heat resistance. However, the yellowing of the sealing material or peeling between members may become evident by the light or heat degradation of the covering material when the module is exposed to the outdoors over 10 years. The yellowing of the sealing material induces a decrease in the amount of incident light and lower an electric output. Meanwhile, peeling between members allows moisture to enter the peeled portion, thereby inducing the corrosion of the photovoltaic element itself or metal members attached to the element, with the result of lowering the performance of the solar cell.
Although the conventionally employed EVA is an excellent sealing material as described above, it is gradually degraded by hydrolysis or pyrolysis. That is, the EVA essentially has such a characteristic that acetic acid is easily liberated therefrom. The liberation of acetic acid occurs due to heat or moisture, thereby causing the yellowing of the sealing material or lowering the mechanical strength or adhesive strength of the sealing material. Furthermore, the liberated acetic acid serves as a catalyst to further promote the degradation. Meanwhile, there is also the problem that the photovoltaic element itself or various metal members attached to the element such as electrodes are corroded by acetic acid.
To solve such problems, a sealing material for the solar cell which has higher degree of durability than its conventional counterparts has been proposed. Japanese Patent Application Laid-Open No. 7-302926 discloses that by using a copolymer resin of ethylene and an unsaturated fatty acid ester as a sealing material, heat resistance and moisture resistance can be improved and degradation in the performance of the photovoltaic element due to the liberation of acid can be inhibited. In particular, the above application discloses that by using the copolymer resin of ethylene and an unsaturated fatty acid ester as a sealing material to a photovoltaic element having a thin semiconductor film and a transparent conductive layer formed on a conductive metal substrate, a short circuit in the element and the corrosion of the transparent conductive layer by an acid can be prevented and a highly reliable solar cell module can be provided.
Further, Japanese Patent Application Laid-Open No. 7-202236 discloses that as the adhesive layer, there can be used an ionomer resin, an ethylene-ethyl acrylate copolymer (EEA) resin, an ethylene-methacrylic acid random copolymer (EMAA) resin, an ethylene-acrylic acid copolymer (EAA) resin, an ethylene-methyl methacrylate copolymer (EMMA) resin, an ethylene-methyl acrylate copolymer (EMA) resin and other adhesive polyolefin resins.
However, the copolymer resin of ethylene and an unsaturated fatty acid ester essentially has low adhesion to metals, glass and other plastic materials, and when it is used as a sealing material for the solar cell module, it is liable to peel off from the element, the front surface member or the back surface member. To overcome such inconvenience, it has been heretofore practiced that an adhesion-improving agent is added to the sealer resin or that the sealer resin is subjected to an easy-adhesion treatment. To state more specifically, the addition of a silane coupling agent or an organic titanate compound to the sealer resin is disclosed in Japanese Patent Publication No. 6-35575 and Japanese Patent Application Laid-Open No. 9-36405, while the treatment of the surface of the front surface member which is in contact with the sealer resin with corona, plasma, ozone, ultraviolet, electron-beam radiation or flame when the front surface member is a film is disclosed in Japanese Patent Application Laid-Open No. 9-36405.
However, the silane coupling agent and the organic titanate compound are susceptible to hydrolysis. Therefore, when a sealer resin to which these have been added in advance is formed into a sheet and thermally crimped to produce a solar cell module, the silane coupling agent or organic titanate compound contained in the sheet reacts with moisture in the air and gradually loses activity during the storage of the sheet, whereby the effect of improving adhesion is liable to be insufficient. Further, when the solar cell module is exposed to the outdoors, the silane coupling agent or organic titanate compound which maintains adhesion among the members is gradually decomposed and loses adhesive strength under the influence of light, heat or moisture. On the other hand, the surface treatment with corona, plasma or the like, generates functional groups on the inactive film surface and forms mainly hydrogen bonds between the film and the sealer resin to improve adhesive strength, but the surface treatment fails to provide a satisfactory effect when the resin substantially lacks adhesion due to such reasons as the lack of polar groups in the sealer resin.
Meanwhile, the copolymer resin of ethylene and an unsaturated fatty acid provides higher adhesion but insufficient thermal stability. Recently, a building material combination type solar cell module, which is obtained by applying a photovoltaic element directly to a building material, is under rapid development, as a solar cell module for electric power which is mounted on the roof or wall of a house, because it is advantageous in view of appearance, applicability and costs. Since less air pass under such a module than a conventional mounted-on-a-base type module, the temperature of the module is elevated to 80xc2x0 C. or higher. At such a high temperature, the copolymer resin of ethylene and an unsaturated fatty acid is gradually decomposed, and yellowing and peeling occur.
It is the object of the present invention to minimize the degradation of the performance of a solar cell during its long-time exposure to the outdoors, especially under high-temperature and high-humidity conditions, by providing a sealer resin for a solar cell module which is excellent in weather resistance, heat resistance and light permeability, poor in water absorption properties, free from the liberation of acid even in the presence of moisture and excellent in adhesion to a various base members, so as to overcome the above problems.
The present inventors have made intensive studies to solve the above problems and found that the following method is the most effective.
That is, in a solar cell module in which the light-receiving surface and/or non-light-receiving surface of a photovoltaic element is sealed by organic polymeric resin layers, at least one of the organic polymeric resin layers comprises an ethylene-unsaturated fatty acid ester-unsaturated fatty acid terpolymer as a main component.